The present invention relates to capacitive energy storage devices for use at cryogenic temperatures, and in particular to a capacitive energy storage device employing specialized electrode layer materials, a method of manufacturing such a device, and a method of charging such a storage device.
Storing energy in banks of capacitors at room temperature is commonly used in applications where size or weight has not been a major concern. Such capacitor technology is quite advanced. However, where the size and weight of the capacitive energy storage devices are of significance and are desired to be minimized, and where rapid discharge is desired, there are potential benefits to be obtained by capacitive energy storage at cryogenic temperatures.
These potential benefits are due to the fact that dielectric breakdown field strengths of dielectric materials used in capacitors are generally much larger at low temperatures than at room temperatures. Also, the dissipation factor for such dielectric materials generally decreases with decreasing temperature so that dielectric heating is reduced in charge-discharge operations. The dissipation factor is a measure of internal power losses due to electronic conduction through the dielectric. This power loss results in the thermal dissipation of electrical energy which is undesirable because it raises the temperature of the device and degrades its efficiency. The resistivity of metals falls rapidly with decreasing temperature so that Joule heating in metal components is reduced during discharge at cryogenic temperatures. Finally, the thermal conductivity of ceramics increases with decreasing temperatures so that heat transfer within components is improved at cryogenic temperatures.
In capacitive energy storage devices, the Helmholtz free energy density of the dielectric is an important quantity. The larger the Helmholtz free energy density of the dielectric, the greater the energy per unit volume which can be stored. The Helmholtz free energy density is defined by the following equation:             Δ      ⁢              xe2x80x83            ⁢      F        =                  1                  8          ⁢          n                    ⁢                        ∫          0                      E            c            2                          ⁢                  ϵ          ⁢                      xe2x80x83                    ⁢                      ⅆ                                          E                2                            ⁡                              (                cgs                )                                                          ,
where xcex94F is the Helmholtz free energy density, ∈ is the dielectric constant of the material, E is the electric field strength, and Ec is the upper limit of electric field strength.
Some studies of capacitive energy storage at cryogenic temperatures have been published. One study dealt with the impregnation of dielectric films with liquid nitrogen or polar liquids. K. N. Mathes and S. H. Minnich, xe2x80x9cCryogenic Capacitor Investigation,xe2x80x9d Final Report, S-67-1095, May 1965. Three types of materials were investigated at 77 K, and it was concluded that energy densities of approximately 0.6 J/cm3 were possible. Energy density may be defined as the energy per unit volume of a medium.
The use of strontium titanate glass ceramic materials as capacitive energy storage devices at cryogenic temperatures was reported by Lawless, Proc. XIII Int""l. Congress of Refrigeration, Washington, D.C., 1971, Vol. 1, p. 599. Based on measurements of electric field strength and dielectric breakdown at 77 K, it was predicted that energy densities of approximately 5.0 J/cm3 were possible.
However, there is a need in the art for materials which can be used as capacitive energy storage devices and which have even greater energy densities. The size and weight of capacitive energy storage devices could be reduced, providing portability to devices which have been heretofore too large and bulky to be mobile. For example, high powered lasers require massive capacitor banks which are too large and heavy to be moved easily. Capacitive devices having large energy densities could reduce the necessary bulk of the capacitors presently utilized in such applications.
U.S. Pat. No. 4,599,677, CAPACITIVE ENERGY STORAGE DEVICE FOR USE AT CRYOGENIC TEMPERATURES, issued Jul. 8, 1986, the disclosure of which is incorporated herein by reference, teaches a capacitive energy storage device utilizing the following ferroelectric pyrochlore ceramic material as the dielectric:
(Cd1xe2x88x92xPbx)2(Nb1xe2x88x92yTay)2O7.
Alternatively the following non-pyrochlore dielectric materials were identified:
xe2x80x83(Sr1xe2x88x92aBaa)TiO3 and (Pb1xe2x88x92bNib)3MgNb2O9.
These ceramic materials were found to possess unusually large dielectric constants at temperatures in the range of about 50 K to 90 K.
However, even in view of the significant advances introduced by the capacitive energy storage devices described in U.S. Pat. No. 4,599,677, there exists a continuing demand for energy storage devices having improved operating characteristics.
This demand is met by the present invention wherein a capacitive energy storage device is provided comprising specialized electrode layer materials selected form the group consisting of YBa2Cu3Ox, and Bi2Ca2Sr2Cu3Oy.
In accordance with one embodiment of the present invention, a capacitive energy storage device for use at cryogenic temperatures is provided comprising first and second electrode layers having a layer of dielectric material there between. The electrode layers comprise an electrically conductive material having a formula selected from the group consisting of: YBa2Cu3Ox and Bi2Ca2Sr2Cu3Oy.
Preferably, the electrically conductive material of the electrode layers is selected such that it is capable of functioning as a superconductor at temperatures in the range of about 50 K to about 90 K. Further, the dielectric material preferably exhibits an improved, or a maximum, dielectric constant at temperatures in the range of about 50 K to about 90 K, relative to the dielectric constant of the dielectric material at room temperature.
The dielectric material may have a formula selected from the group consisting of: (Cd1xe2x88x92xPbx)2(Nb1xe2x88x92yTay)2O7, where x and y are values between about 0 and about 1, (Sr1xe2x88x92aBaa)TiO3, where a is a value between about 0 and about 0.5, and (Pbbxe2x88x921Nib)3MgNb2O9, where b is a value between about 0 and about 1. Additionally, the dielectric material may be a combination of (Sr1xe2x88x92aBaa)TiO3(x), where a is a value between about 0 and about 0.5, and (Pb1xe2x88x92bNib)3MgNb2O9(1xe2x88x92x), where b is a value between about 0 and about 1, and x is a mole percentage value between about 5 and about 50, and preferably, from about 40 to about 50. Likewise, capacitors made with the ceramic materials of the preferred embodiment of the present invention possess large energy densities, i.e., approximately 20-25 J/cm3, at temperatures near 77 K (the temperature of liquid nitrogen).
In accordance with another embodiment of the present invention, a method of producing a capacitive energy storage device for use at cryogenic temperatures is provided. The method comprises providing a layer of dielectric material having a pair of opposite substantially parallel major faces, and providing a first electrode layer on one of the major faces. The method further comprises providing a second electrode layer on the other of the major faces, wherein the electrode layers comprise an electrically conductive material, and co-firing the dielectric layer, the first electrode layer, and the second electrode layer for a duration and at a temperature sufficient to sinter the dielectric layer. Preferably, the temperature is between about 950xc2x0 C. and 1100xc2x0 C. Further, the steps of providing the first and second electrode layers may comprise providing an electrode layer material slurry. Moreover, the electrically conductive material, for example, and not limited to, may having a formula selected from the group consisting of: YBa2Cu3Ox, Bi2Ca2Sr2Cu3Oy, and the like.
In accordance with yet another embodiment of the present invention, a capacitive energy storage device for use at cryogenic temperatures is provided comprising: (i) a plurality of electrode layers having at least one layer of dielectric material positioned between respective electrode layers; (ii) additional layers of dielectric material positioned to define respective exterior major faces of the storage device; and (iii) a thermally conductive heat dissipation pad bonded to at least one of the exterior major faces. Preferably, the thermally conductive heat dissipation pad comprises a silver dot.
In accordance with yet another embodiment of the present invention, a method of storing electrical charge in a capacitive energy storage device is provided. The method comprises the steps of: (i) reducing the temperature of the capacitive energy storage device from a predetermined room temperature to a predetermined cryogenic operating temperature; (ii) applying a first electric field of a first field strength across the dielectric layer, wherein the magnitude of the first field strength is sufficient to cause the storage device to switch from a first operational state to a second operational state, the first operational state is characterized by a first dielectric constant, the second operational state is characterized by a second dielectric constant, and the second dielectric constant is substantially greater than the first dielectric constant; and (iii) charging the capacitive energy storage device in the second operational state.
The capacitive energy storage device may be charged in the second operational state by applying a second electric field of a second field strength across the dielectric material after application of the first electric field across the dielectric material. The first electric field may be applied such that the capacitive energy storage device may be charged in the second operational state regardless of the strength of the second electric field. The second dielectric constant is preferably substantially greater than the first dielectric constant at the second field strength and at the first field strength. The predetermined cryogenic operating temperature may be less than approximately 200 K, and is preferably between about 50 K and about 90 K. The first field strength may be approximately 200 kV/cm. The second field strength is typically greater than the first field strength.
In accordance with yet another embodiment of the present invention, a method of producing an improved capacitive energy storage device is provided. The method comprises the steps of: (i) reducing the temperature of the layer of dielectric material from a predetermined room temperature to a predetermined cryogenic operating temperature; and (ii) applying a first electric field of a first field strength across the dielectric layer, wherein the magnitude of the first field strength is sufficient to cause the dielectric layer to switch from a first operational state to a second operational state, the first operational state is characterized by a first dielectric constant, the second operational state is characterized by a second dielectric constant, and the second dielectric constant is substantially greater than the first dielectric constant.
In accordance with yet another embodiment of the present invention, an improved capacitive energy storage device is provided comprising first and second electrode layers having a layer of dielectric material there between, wherein said dielectric material has the formula:
(Cd1xe2x88x92xPbx)2(Nb1xe2x88x92yTay)2O7
where x and y are values between about 0 and about 1, and wherein said capacitive energy storage device is produced by (i) reducing the temperature of the layer of dielectric material from a predetermined room temperature to a predetermined cryogenic operating temperature; and (ii) applying a first electric field of a first field strength across the dielectric layer, wherein the magnitude of the first field strength is sufficient to cause the dielectric layer to switch from a first operational state to a second operational state, the first operational state is characterized by a first dielectric constant, the second operational state is characterized by a second dielectric constant, and the second dielectric constant is substantially greater than the first dielectric constant.
Accordingly, it is an object of the present invention to provide a capacitive energy storage device for use at cryogenic temperatures having improved operational characteristics through proper selection of a material for forming the electrode layers of the device and proper selection of a storage device manufacturing process. It is a further object of the present invention to provide an improved method of charging such a charge storage device. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.